diff --git a/src/assets/data/bodies.json b/src/assets/data/bodies.json
index bfc18e2..55b0051 100644
--- a/src/assets/data/bodies.json
+++ b/src/assets/data/bodies.json
@@ -707,7 +707,7 @@
       "raDeg": 317.671,
       "decDeg": 52.893
     },
-    "orbitSource": "JPL SSD satellite mean elements, epoch 1950 Jan 1, within 1.3 degrees of Horizons from 1950 to 2100",
+    "orbitSource": "JPL SSD satellite mean elements, epoch 1950 Jan 1, with the orbital terms of its IAU W (NAIF pck00011), within 1.3 degrees of Horizons from 1950 to 2100",
     "parentBodyId": "mars",
     "rotationPeriodHours": 7.653844882637157,
     "rotationalElements": {
@@ -1184,7 +1184,7 @@
       "raDeg": 40.589,
       "decDeg": 83.536
     },
-    "orbitSource": "JPL SSD satellite mean elements, epoch 2000 Jan 1, within 7.5 degrees of Horizons from 1950 to 2100",
+    "orbitSource": "JPL SSD satellite mean elements, epoch 2000 Jan 1, with the orbital terms of its IAU W (NAIF pck00011), within 7.5 degrees of Horizons from 1950 to 2100",
     "parentBodyId": "saturn",
     "rotationPeriodHours": 22.618127008672275,
     "rotationalElements": {
@@ -1300,7 +1300,7 @@
       "raDeg": 40.578,
       "decDeg": 83.537
     },
-    "orbitSource": "JPL SSD satellite mean elements, epoch 2000 Jan 1, within 0.3 degrees of Horizons from 1950 to 2100",
+    "orbitSource": "JPL SSD satellite mean elements, epoch 2000 Jan 1, with the orbital terms of its IAU W (NAIF pck00011), within 0.3 degrees of Horizons from 1950 to 2100",
     "parentBodyId": "saturn",
     "rotationPeriodHours": 45.30726088829954,
     "rotationalElements": {
diff --git a/tools/etl/build.ts b/tools/etl/build.ts
index ac6bc65..bd1d8c0 100644
--- a/tools/etl/build.ts
+++ b/tools/etl/build.ts
@@ -181,9 +181,13 @@ const MOON_OFFSET_CEILINGS_DEG: Record<string, number> = { hyperion: 21, iapetus
  *   `orbitFromW` in `fetchSolarSystem.ts`), but not the rest of what Horizons integrates.
  * - Ceres, 7.12 (1953): the SBDB's elements are osculating, exact at 2026 Jun 9 and drifting
  *   either side; 1.9 by 2050, 5.3 by 2100, and 39 at 1600 on Horizons' own figures.
+ *
+ * And one is held tighter than the rest: Tethys, 0.28, which takes the other half of that
+ * libration, 2.23 degrees, from its W. Without it Tethys strays 2.09 degrees, under the general
+ * ceiling, and its card would quietly restate itself as "within 2.1".
  */
 const MAX_TRACK_OFFSET_DEG = 3;
-const TRACK_OFFSET_CEILINGS_DEG: Record<string, number> = { hyperion: 23, nereid: 12, iapetus: 11, mimas: 8, ceres: 8 };
+const TRACK_OFFSET_CEILINGS_DEG: Record<string, number> = { hyperion: 23, nereid: 12, iapetus: 11, mimas: 8, ceres: 8, tethys: 0.5 };
 
 /**
  * The bodies the IAU WGCCRE 2015 report gives no rotational elements for: Hyperion tumbles, and
@@ -222,8 +226,13 @@ const MAX_OBLIQUITY_OFFSET_DEG = 0.1;
  * sampled every 135 days from 1950 to 2100, where both the tables and the IAU's elements hold.
  *
  * Measured on this catalogue: at most 6.70 degrees (the Moon, whose longitude swings 6.3 either
- * way with its eccentricity; Horizons has the same). Three need their own: Mimas 10.15, whose
- * physical libration W carries and Horizons shows as 5 to 9 degrees at its true place; Iapetus
+ * way with its eccentricity; Horizons has the same). Three need their own. Mimas 10.15: its drawn
+ * face runs from 2.5 to 10.15 degrees, about 6.3 off on average because the IAU's W and JPL's mean
+ * longitude disagree, drifting 3.3 over the span because W turns 6.0e-5 degrees a day faster than
+ * the row's n, and swung 2.3 either way (2e) by its eccentricity. None of that is Mimas: its
+ * measured physical libration is 0.84 degrees (Tajeddine et al. 2014, Science 346, 322), and W
+ * carries none; Horizons, on the same W against its integrated orbit, runs from -2.7 to 12.7
+ * degrees over 1950-2100 with the 71-year S5 term the orbit here cancels. Iapetus
  * 18.33, whose row sits 9.4 degrees behind Horizons; and Proteus 8.18, whose W turns 6.3e-7 of
  * its rate slower than its orbit, a drift of 74 degrees by AD 3000. What this catches is an orbit
  * and a W that go round at different rates: the tidal acceleration W carried and the orbit did not
@@ -283,6 +292,12 @@ function validateBodies(bodies: BodyRecord[], horizonsOrbits: Map<string, Orbita
       (track !== undefined) === !body.orbitSource.startsWith('JPL approximate mean elements (Standish)'),
       `${body.name}'s orbit, "${body.orbitSource}", ${track ? 'names its own span' : 'names no span it holds over'}.`
     );
+    // JPL's satellite table carries no periodic terms: a moon's are from its IAU W, and its card
+    // names the kernel they come from as well as the table.
+    assertCondition(
+      !body.parentBodyId || !body.rates.meanAnomalyTerms || body.orbitSource.includes('NAIF pck00011'),
+      `${body.name}'s orbit carries terms taken from its IAU W, and its card, "${body.orbitSource}", credits only the table.`
+    );
     if (track) {
       const worst = Math.max(...track.map((point) => offsetFromTrackDeg(body, point)));
       const trackCeiling = TRACK_OFFSET_CEILINGS_DEG[body.id] ?? MAX_TRACK_OFFSET_DEG;
diff --git a/tools/etl/fetchSolarSystem.ts b/tools/etl/fetchSolarSystem.ts
index 9e872cf..259dc2d 100644
--- a/tools/etl/fetchSolarSystem.ts
+++ b/tools/etl/fetchSolarSystem.ts
@@ -221,8 +221,9 @@ export async function fetchSolarSystem(): Promise<{ bodies: BodyRecord[]; horizo
 
     // Standish's fit states its own span, 3000 BC to AD 3000. The moons' table and the SBDB state
     // none, and hold for far less: each card says how far its orbit stays from Horizons over the
-    // span it was measured, where the clock reaches AD 1 to AD 3000.
-    let orbitSource = mean.orbitSource;
+    // span it was measured, where the clock reaches AD 1 to AD 3000. An orbit that took terms from
+    // its IAU W says so first: they move Mimas by up to 44.85 degrees, and are none of JPL's table.
+    let orbitSource = fromW ? `${mean.orbitSource}, with the orbital terms of its IAU W (NAIF pck00011)` : mean.orbitSource;
     if (parentName || smallBody) {
       const stepDays = spec.trackStepDays ?? 2;
       const track = await fetchHorizonsTrack(spec.horizonsCommand, spec.center, stepDays, `horizons-track-${spec.id}-${stepDays}d.txt`);